Custom tile size

[mjuez]

Setting the tile size for each slice is mandatory, because different slices has different dimensions and smallest tile size has to be different for each one.

The problem occurs when setting custom tile size (different to 256).
With tile size = 256, when zoom = 0, x and y coords are always 0 (one tile).
With tile size != 256, when zoom = 0, x and y takes values 0 and 1 (four tiles), which is not the desired behavior.